The invention relates to an expanding body which extends across the width of a machine (e.g. a papermaking or converting machine or a similar machine) and is used on this for changing the position of a machine element, e.g. strip, profile bar or the like. The walls of the expanding body shaped similar to a box are formed from a flexible material; they confine an inner chamber which can be pressurized by a medium so that at least one of the walls, viewed in the cross-section through the expanding body, is expanded in the longitudinal direction.
Such an expanding body has been proposed in Federal Republic of Germany Patent Application Serial No. P 37 10 058.0. It is arranged at the discharge opening of a headbox and carries a blade-like bar, which is slidable at right angles to the stock flow for the purpose of altering the clear width of the discharge opening. The sliding of the bar is effected in that the inner chamber of the expanding body can be pressurized with variable pressure by a pressure medium. Viewed in a cross-section, the expanding body has four walls, which confine the inner chamber. Two opposing walls extend in the direction of movement of the bar parallel to each other. These two walls and the cross wall arranged at a distance from the bar are supported from the outside by rigid components. The cross wall located in the vicinity of the bar can be stiffened by an armoring. In this case, the walls parallel to each other have a higher expandability than the stiffened cross wall, which is free on its outside, i.e. not supported. By variation of the pressure prevailing in the inner chamber of the expanding body the two walls parallel to each other can expand more or less in the direction of movement of the bar, whereby the externally free cross wall is adjusted together with the bar. Since the inner chamber is subdivided by means of a plurality of cross walls into several sections which are disposed one behind the other in machine cross direction, and since each section can be pressurized individually by a variable fluid pressure, the bar can be adjusted by different amounts from section to section.
A disadvantage of the proposed design is that the two highly expandable walls parallel to each other must slide along the rigid components supporting them. This generates friction forces at the sliding surfaces This makes it very difficult to precisely pre-determine the position change of the bar resulting from a well-defined change of the internal pressure.